A kinetic study of the reactions of iron oxides and hydroxides relevant to the chemistry of iron in the upper mesosphere

Abstract

This paper describes the kinetic study of a number of gas-phase reactions of iron oxides and hydroxides with O, H and O₃. These reactions are important for characterising the chemistry of meteor-ablated iron in the earth's upper mesosphere. Pulses of atomic Fe were produced in the upstream section of a fast flow tube by the pulsed laser ablation of a pure Fe rod, and detected at the downstream end by LIF at 248.3 nm Fe(x⁵F⁰₅ ← a⁵D₄). The Fe-containing reactant species FeO and FeO₂ were produced by sequential reaction of Fe with NO₂; FeO₃ by the reaction of metastable excited Fe atoms with O₂ to form FeO, followed by addition of O₂; and Fe(OH)₂ by the addition of H₂O to FeO. Atomic O or H was produced by the microwave discharge of N₂ (with addition of NO) or H₂, respectively, and their absolute concentrations determined by conventional titration with NO₂. Rate coefficients were essentially measured relative to absolute rate coefficients for Fe and FeO determined previously [J. M. C. Plane and R. J. Rollason, Phys. Chem. Chem. Phys, 1999, 1, 1843; R. J. Rollason and J. M. C. Plane, ibid., 2000, 2, 2335], but were extracted using a full kinetic model including diffusive loss on the flow tube walls of the relevant species. The following results were obtained (units: cm³ molecule⁻¹ s⁻¹; quoted uncertainty is 2σ): k(FeO + O → Fe + O₂, 209–381 K) = 4.6^+2.6_−1.6 × 10⁻¹⁰ e^{−(350±130)/T}; k(FeO₂ + O → FeO + O₂, 209–381 K) = 1.4^+0.8_−0.5 × 10⁻¹⁰ e^{−(580±120)/T}; k(FeO₃ + O → FeO₂ + O₂, 610 K) = 8⁺¹⁰₋₅ × 10⁻¹²; k(FeO₂ + O₃ → FeO₃ + O₂, 224–298 K) = 4.4^+6.4_−2.6 × 10⁻¹⁰ e^{−(170±230)/T}; k(FeO₃ + H → FeOH + O₂, 294 K) = (2.0^+1.2_−0.6) × 10⁻¹¹, and k(FeOH + H → products, 294 K) = (1.3 ± 0.3) × 10⁻¹¹ . Theoretical calculations at the B3LYP/6-311 + g(2d,p) level were used to identify the stationary points on the relevant potential energy surfaces for most of these reactions, before applying statistical theories to model the kinetics. The implications of these results for the chemistry of iron in flames and the upper atmosphere are then discussed.